(1) Field of the Invention
The present invention relates to an optical pickup device in which a tracking error of a main spot on an optical disk is detected in response to reflection beams of side spots on the disk when information is read from or written to the disk, so as to trail a track of the disk at its exact position.
(2) Description of the Related Art
When an optical disk is inserted into an optical disk drive, the optical disk drive reads information from or writes information to the optical disk by using an optical pickup device. The optical pickup device emits a light beam to the disk, and detects a tracking error from reflection beams from the disk to which the emission beam is applied, in order to trail a track of the disk at the exact position. When the optical disk drive is operating in a reading mode, the optical pickup device detects pits in the track of the disk from the reflection beams in order to read data from the disk.
As described above, in the optical pickup device, a tracking error detection is carried out. The tracking error detection allows the emission beam to trail a track of the optical disk to the exact position when recording, playing or erasing information on the optical disk.
There are two principal types of optical disk drive: a read-only type and a rewritable type. The read-only optical disk drive reads information from an optical disk, such as CD (compact disk) and CD-ROM (compact disk-read-only memory). The rewritable optical disk drives can read information from and write information to an optical disk. Among the rewritable optical disk drives are a write-once disk system and an erasable disk system. The write-once disk system accesses an optical disk such as CD-R (compact disk-recordable), and can write data to the disk once only. The erasable disk system accesses an optical disk, such as PD (phase-change material disk) or CD-RW (compact disk-rewritable), and can write data to the disk many times.
There are three known methods of tracking error detection used by the existing optical disk drive: (1) a push-pull method, (2) a three-spot method, and (3) a differential push-pull method.
In the push-pull method, a single light beam emitted by a light source is used. The emission beam from the light source is directed to a track of the disk. A two-division photodetector receives reflection beams from the disk and outputs signals indicating intensities (or amounts of light) of the reflection beams. A tracking error signal is generated based on the signals output from the two-division photodetector.
When the push-pull method is used, an objective lens directing the emission beam to the disk is shifted in a radial direction of the disk, the reflection beams directed from the disk to the two-division photodetector are shifted. In this case, a tracking error signal having an offset caused by the radial shift of the objective lens is generated based on the signals output from the two-division photodetector. Even if a spot is located at the center of the track of the disk, the tracking error signal which is not equal to zero may be generated according to the push-pull method.
In the three-spot method, three spots (or a main spot and a pair of side spots) are arranged on the disk by a light beam emitted by a light source. Two different photodetectors individually receive reflection beams of the side spots on the disk, and each of the two photodetectors outputs a signal indicating the amount of light of a corresponding one of the reflection beams. A tracking error signal is generated based on a difference between the amounts of light indicated by the signals output from the two photodetectors.
When the three-spot method is used, the offset caused by a radial shift of the objective lens in the case of the push-pull method is not produced. However, when data is written to a blank area of a write-once optical disk or an erasable optical disk, the preceding one of the side spots to the main spot is located at the blank area in the disk and the following one of the side spots to the main spot is located at the recorded area in the disk. As the blank area and the recorded area in the disk have different reflectivities, the reflection beams directed from the disk to the photodetectors indicate a difference in the amount of light due to the different reflectivities. In such a case, a tracking error signal having an offset is generated based on the output signals of the two photodetectors. Even if the main spot is located at the center of the track of the disk, the tracking error signal which is not equal to zero may be generated according to the three-spot method. Therefore, the three-spot method is used only in an optical pickup device of the read-only type disk drive such as a CD-ROM drive or a MO (magneto-optical) disk drive.
In the differential push-pull method, a main spot and a pair of side spots are arranged on the disk by a light beam emitted by a light source. By using the push-pull method, a tracking error signal TE1 is generated based on the signals output in response to the reflection beam of the main spot on the disk, and two tracking error signals TE2 and TE3 are generated based on the signals output in response to the reflection beams of the side spots on the disk.
According to the differential push-pull method, a tracking error signal TE at an output of an optical pickup device is generated from the tracking error signals TE1, TE2 and TE3 in accordance with the following formula: EQU TE=TE1-k(TE2+TE3) (1)
where k is a coefficient, TE1 is the tracking error signal for the reflection beam of the main spot, and TE2 and TE3 are the tracking error signals for the reflection beams of the side spots.
When the differential push-pull method is used, each of the tracking error signals TE1, TE2 and TE3 contains the offset caused by a radial shift of the objective lens in the case of the push-pull method, but the calculation of the above formula (1) eliminates the offset caused by the radial shift of the objective lens. Therefore, the tracking error signal TE output by the optical pickup device does not contain the offset caused by the radial shift of the objective lens.
However, when data is written to a blank area of a write-once optical disk or an erasable optical disk under a condition in which the objective lens is shifted in a radial direction of the disk, the differential push-pull method does not provide an accurate tracking error detection. As the blank area and the recorded area in the disk have different reflectivities, the reflection beams directed from the disk to the photodetectors indicate a difference in the amount of light due to the different reflectivities. In such a case, a tracking error signal TE having an offset is generated from the tracking error signals TE1-TE3 output by the photodetectors. Even if the main spot is located at the center of the track of the disk, the tracking error signal which is not equal to zero may be generated by the differential push-pull method. The differential push-pull method differs from the three-spot method in that the differential push-pull method can accurately detect a tracking error signal if the objective lens is not shifted in the radial direction of the disk.
Further, when the above-mentioned tracking error detection methods are used, a tracking error signal having a DC offset and an offset caused by the phase shift may be generated when data is written to a blank area of a write-once optical disk or an erasable optical disk under a condition in which the objective lens is shifted in a radial direction of the disk. It is difficult for the above-mentioned tracking error detection methods to provide an accurate tracking error detection in such a case.